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Producer is a much more general term. A manufacturer can manufacturer OR produce something. A producer doesn't necessarily have to manufacture something. Shadow is right -- all those things are produced, but it's just as useful a term for a factory or whatever. It's perfectly normal for instance to say "The local tractor factory produces three hundred tractors a month."

One more question: I'm quite unsure about this sentence : "As I have written earlier, they already have been added! " - It was written by my colleague....
I think It must be "As I
wrote earlier, they already have been added! "

The difference is not too well defined here; they can mean the same thing (although as people already noted 'producer' is more general).
If they are used in the same text to differentiate things, my guess (not knowing the context) would be that 'manufacturer' is the company that is actually running the production plant, and 'producer' is the company that is selling the product (they are quite often different).
But I could be wrong.

One more question: I'm quite unsure about this sentence : "As I have written earlier, they already have been added! " - It was written by my colleague....
Boots Lining Closed Ruched Closure Waterproof Toe Boots Ground Heel Manmade Ground 1TO9 Brown MNS02320 Hard Womens Manmade No Urethane High Toe Pointed Firm Warm I think It must be "As I
wrote earlier, they already have been added! "

Both are fine, as far as I know. I don't really know how to explain it.

And just FYI, "oughta" is of course very informal -- I'm not sure if you realized that or not. It's really only ok to use in speech -- in written form it's typically just used to quote someone, I'd say (it's a fairly ugly looking word ).
Marc told me "You oughta buy me some cheese too!" Then he pulled out a gun and robbed the clerk.

One more question: I'm quite unsure about this sentence : "As I have written earlier, they already have been added! " - It was written by my colleague....
I think It must be "As I
wrote earlier, they already have been added! "

Who is right ?

You are definately correct on this. For "have written" to be correct the second clause would need to be in the present tense. "As I have written earlier, we are all doomed" is ok. The past perfect form implies relevance to the present. Simple past goes with past.